1. Field of the Invention
This invention relates to locking arrangements in general and in particular to an alarm system incorporated in a lock structure for producing an alarm signal upon an attempt to open the lock structure through other than use of a designated key.
2. Discussion of Related Art
Many pick-proof and alarmed locks have been suggested in the past. For instance, U.S. Pat. No. 2,011,944, issued Aug. 20, 1935, to Lodispoto, shows a combined lock and switch for an alarm wherein a lock body has a key slot formed wider than the key. Grooves are formed in the sides of the key slot and coact with transverse ribs which extend for the length of the key. Pins extend into the key slot and if a false key is inserted into the slot it will contact one of the pins. The proper key will be guided by the transverse ribs and avoid the pins.
U.S. Pat. No. 2,874,240, issued Feb. 17, 1959, to Ricks, shows an automatic door-opening signal device wherein a sleeve is placed over a door knob. The sleeve has a cam surface which contacts a switch upon rotation of the door knob and the sleeve. The switch in turn actuates an alarm when an attempt is made to open the door.
U.S. Pat. No. 3,660,624, issued May 2, 1972, to Bell, shows an electrical key for ignition systems. The Bell device includes a key having adapters that are irremovably interposed between a predetermined number of spark plug cable receptacles and their respective distributor contacts. The electrical key includes an electrical coded key element which works in conjunction with an electrical coded key adapter when the key element is inserted to produce an electrical continuity in the vehicular ignition system.
U.S. Pat. No. 3,797,004, issued Mar. 12, 1974, to Muessel et al., shows a lock tumbler cylinder picking alarm. A shutter is mounted on a front portion of a key plug that rotates in a lock tumbler cylinder, and an electric switch on the cylinder has a spring contact arm pressing the shutter to a position closing the entrance opening of the keyway. A key or other instrument inserted toward the keyway must move the shutter so that the spring arm will complete an electric circuit causing an alarm signal before the instrument can engage any one of the lock tumblers. The shutter will move away from the spring arm when the key plug rotates so that the alarm signal will be brief when a proper key is used, and a short time delay in the alarm circuit then will withhold operation of an alarm.
U.S. Pat. No. 3,986,376, issued Oct. 19, 1976, to Lack, shows a pin and tumbler type lock in which a slide element is disposed at the end of the cylinder opposite the end from which the key is inserted. The slide element cooperates with a pin in the barrel and has a projecting portion diametrically opposite to the pin with which it cooperates which, in the locked condition, engages in an opening of the barrel. The slide element has a configuration for cooperation with the leading end of a key so that insertion of the correct key causes disengagement of that portion of the slide extending into the opening in the barrel and additionally brings abutting portions of the slide element and the cooperating pin to a shear line between the cylinder and the barrel to free the lock.
U.S. Pat. No. 4,061,004, issued Dec. 6, 1977, to Pappanikolaou, shows a pick-proof lock cylinder having a keyhole which opens to provide access to the cylinder. Two cam surfaces are provided on the remote end of the keyhole configuration to deflect a pair of coded, pivotally mounted fingers on a jointed key which is receivable within the keyhole. The fingers of the key may be mechanically or magnetically coded to cause the enabling pins or plungers to be actuated when the key is fully inserted into the keyhole, and the coded fingers are deflected to their operative positions by the cam surfaces.